Small molecules can play roles as intracellular messengers for signaling pathways within the human body. For example, nitric oxide (NO) can participate in signaling in the cardiovascular and nervous systems, and can be employed in the human immune response system. Detection of small molecules has traditionally been relatively difficult, and becomes even more difficult at low concentrations. Examples of tools that may be used to detect such species include, for example, visible-fluorescence probes, chemiluminescence-based devices, and X-ray photoelectron and electron paramagnetic resonance (EPR) spectroscopy. For example, in the case of NO, a series of diaminofluoresceins and metal-fluorophore complexes have been widely applied to detect cellular NO. However, such methods may include significant limitations. For example, diaminofluoresceins generally detect molecules indirectly (e.g., via oxidation products). Other limitations include photobleaching and lack of optical penetration through biological tissues for metal-fluorophore complexes. Therefore, the design of more robust schemes for the biological detection of relatively small molecules is still an active area of research. Nanotechnology has produced several new classes of biosensors, but their extension to in vivo application has been limited.